This invention relates to AFC (automatic frequency control) circuits in TV (televison) tuners, and more particularly to an AFC circuit in a color TV tuner, which performs AFC with an audio carrier signal.
An AFC circuit in a color TV tuner and especially in an audio tuner employs an audio carrier signal to effect automatic frequency control in view of the S/N ratio and the manufacturing cost. That is, a detection output according to the amount of detuning of the received signal frequency is employed as the AFC voltage, which is applied to the vari-cap in the local oscillating circuit, to achieve automatic frequency control.
Immediately after the power switch is turned on or when the channel is changed, the local oscillating frequency is unstable. Accordingly, sometimes the received signal frequency is shifted by several hundred KHz to 1 MHz from the tuning point, and is thereafter returned to the tuning point by the operation of the AFC circuit. In this connection, it should be noted that the difference in frequency between the audio carrier and the color carrier is only 920 KHz. Therefore, if the tuning is shifted by about 500 KHz or more towards the color carrier when the power switch is turned on or when the channel is changed, the AFC will be applied to the color carrier; that is, so-called "mislocking" takes place, thus interrupting correct signal reception.